This invention relates to a heat storage material comprising calcium chloride-hexahydrate as the main component.
Since calcium chloride-hexahydrate (CaCl.sub.2.6H.sub.2 O, m.p. 30.degree. C.) is inexpensive and has large latent heat, it is a hopeful material as a latent-heat storage material for hothouses and heat-pump type air conditioners. However, it seriously undergoes a supercooling phenomenon at the time of heat release and does not release latent heat even if its temperature is lowered to a temperature of 20.degree. C. or more lower than the solidifying point. Therefore, it cannot be used as a heat storage material unless its supercooling phenomenon undesirable for a heat storage material is avoided.
In U.S. Pat. No. 4,189,394 is disclosed a heat storage material containing, as a nucleating agent for avoiding the supercooling phenomenon, one or more compounds selected from the group consisting of barium carbonate, strontium carbonate, barium fluoride, barium fluoride-hydrofluoride and strontium fluoride.
In Japanese Patent Appln Kokoku (Post-Exam Publn) No. 9596/78 is known, as nucleating agents, one or more compounds selected from the group consisting of anhydrous barium hydroxide, barium hydroxide-octahydrate, anhydrous strontium hydroxide and strontium hydroxide-octahydrate.
Further, in Japanese Patent Appln Kokoku (Post-Exam Publn) No. 9959/81 is disclosed a heat storage material in which a strontium halide is used as a nucleating agent.
However, when these already-known nucleating agents are used, the supercooling temperature of the resulting heat storage material becomes 3.degree. C. or more, or the nucleating agents are molten and lost during a heat cycle in the heat storage material, so that they are poor in reproducibility and change the inherent melting point of the heat storage material.
The heat design for a heat storage tank should after be made under the condition that the difference between the melting point of the heat storage material and the temperature of a heat medium is 3.degree. C. or less at the time of taking out heat from the heat storage material, from the viewpoint of the designs of refrigeration cycle and a heat exchanger (since the melting point of calcium chloride is 30.degree. C., the design is made under the condition that the temperature of the heat medium is 27.degree. C.). That is to say, the difference in temperature between a latent-heat storage material is heat storage capsules and a heat medium (e.g., water) flowing outside the heat storage capsules is permissible up to only 3.degree. C. Considering the presence of the thermal resistance of thick portions of the heat storage capsules and the thermal resistance of the heat medium and the capsules, a temperature difference of about 1.degree. C. or more arises in these portions, so that the temperature of calcium chloride cannot be below 28.degree. C. A heat storage material having a degree of supercooling .DELTA.Ts of 3.degree. C. or more owing to a combination of a nucleating agent and calcium chloride does not release latent heat due to solidification unless its temperature reaches 27.degree. C. which is 3.degree. C. lower than the melting point of calcium chloride of 30.degree. C. At the aforesaid temperature of 28.degree. C., it is permanently kept in a liquid state as it is in a supercooled state, so that it does not permit, as a latent-heat storage material, the withdrawal of effective latent heat. Therefore, it is very important to find a nucleating agent for limiting the degree of supercooling .DELTA.Ts to 2.degree. C. or less. As a nucleation theory, an interfacial-energy theory is now approved. In short, the smaller the interfacial energy between the nucleating agent and the resulting crystal, the easier the formation of crystal nucleus. That is to say, the smaller the contact angle between the nucleating agent and the crystal, the easier the formation of the nucleus.
In order to know whether a nucleating effect is brought about depending upon the interfacial energy or the magnitude of the contact angle, there is now no effective means except an experiment carried out by incorporating, in practice, a nucleating agent into a heat storage material.